Identifying genes and gene products that are important in tumor cells is a high priority goal of the National Cancer Institute. In order to determine whether a gene of interest is expressed in human malignancy, investigators must be able to evaluate actual tissue samples of human tumors. Unfortunately, access to large numbers of well characterized human tissue samples is difficult, and there is a large expence for the preparation of glass slides from tumor in order to perform immunohistochemistry or in-situ hybridization. As a partial solution to this problem, pathologists have tried to combine multiple tissue samples in one tissue block, that could then be used to reduce the histology costs and the time and effort involved to perform the special studies. This technique has been refined to the point where hundreds of tissue samples can be placed in a grid arrangement in a single paraffin tissue block. Investigators in the NHGRI (Kohenen, Kallioniemi and others) demonstrated the utility of large scale tissue microarrays in a seminal paper published in Science. Following a tissue microarray workshop hosted by NCI in Fall, 1999, a steering committee convened with the purpose of establishing a pathology-based tissue microarray core facility. This project is a direct outgrowth of the Extraordinary Opportunity to Define the Signatures of Cancer Cells identified in the 2001 Bypass Budget plan. The goals of the first year were to establish a core microarray production facility based on a similar facility in the NHGRI and to create the first mixed tumor microarrays for nationwide distribution. Space for the core facility was assigned in the Advanced Technology Core and a histotechnologist was hired to handle the technical aspects of establishing the histology laboratory and to perform block cutting and staining activities. The steering committee, which is composed of representatives from intramural and extramural NCI and from the NHGRI, determined that the first product of the core should be a mixed tumor block containing representative samples of the most common epithelial malignancies (breast, colon, lung, prostate, and ovary) as well as samples of melanoma, glioma and lymphoma. A selection of normal tissue and standard cell lines were also to be included to bring the total number of tissue spots to approximately 700. Tumor samples were obtained through the CHTN, and this organization will also handle distribution of arrays on glass slides to investigators in the intramural and extramural scientific community. The current target date for the first multitumor arrays is November, 2000 and arrayed slides will be distributed for beta testing. The steering committee will then make decisions about future tumor blocks that the core will prepare, but options include both specialty blocks of rarer tumors (renal cell, hepatocellular, sarcomas), blocks prepared from collections linked to prognosis (e.g. the NSABP collection of breast cancers) or blocks specific to particular clinical trials.